This is the first year my 6 year old wanted to be something scary. He started out wanting to be a ghost but then ultimately decided skeleton. The costume was a bit scary for him, though, because at first he was scared of the mask. He tried to cover it up saying "the other kids might be scared," but once he got it on and saw no one was actaully scared, then he really had fun.
I think babies just default to cute normally. I was debating if he should go as the Incredible Hulk, though, because he has some major temper tantrums already and he's only 10 months old.
I saw a group of zombie kids. They were around 10 years old, two boys and a girl, and the scariest and bloodiest costumes I saw all night.
The idea that we dress as the ghouls to chase them away has pretty much gone from the idea of Halloween. It's more the night to play pretend with everyone else. Many places ban scary costumes now. Some even ban t.v. and movie / princess / superhero costumes and say only "historical" or "cultural" costumes allowed.